feat(serve): shutdown endpoint

[axel7083]

Issue for this PR

Closes #20277

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Provide a /shutdown endpoint to stop the server and opencode exit nicely.

How did you verify your code works?

1. Build & start opencode container

$: cd packages/opencode
$: bun run build
$: podman build -t localhost/opencode:latest -f Containerfile .
$: podman run -it --rm -p 4096:4096 localhost/opencode:latest serve --hostname=0.0.0.0 --port 4096

2. In another terminal curl the /shutdown endpoint

$: curl --location --request POST 'http://0.0.0.0:4096/shutdown'

Assert the server shutdown gracefully.

Screenshots / recordings

N/A

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR